Preventing the initiation, and reducing the escalation of marijuana use among adolescents can have a major impact on the development of problem behaviors and health consequences in adulthood. The varying factors associated with initiation and escalation of marijuana use among adolescents (age 12-18) speaks to the importance of tailoring preventive approaches based on individual risk factors. Brief motivational interventions (BMI) have been demonstrated to be effective in reducing drug use (i.e., marijuana, alcohol, and tobacco) among adolescents and young adults. Typically, BMIs are delivered by a clinician; however, more recently, computers have been used successfully to deliver BMI in an interactive, graphically engaging format. The primary care setting represents an underutilized venue for prevention interventions addressing drug use/abuse among youth. This study will take place in three HRSA-funded Bureau of Primary Health Care community health clinics located in Flint, Michigan, a city with a diverse and medically under-served population. The purpose of this five-year study is to: (1) develop and refine promising, empirically-derived, BMI prevention/intervention modules delivered by a clinician or a computer to target marijuana use, and (2) test the effectiveness of BMI in preventing initiation/escalation of marijuana use among youth in community health clinics. The proposed BMI prevention/interventions are innovative because: a) they incorporate tailoring technology based on assessment of the adolescents' behaviors; and b) they incorporate prevention messages for those who have not initiated use, and intervention messages for those who are current users. This randomized controlled trial will screen 1,710 adolescents in primary care clinics over 30 months. A random sample of the approximately 70% who screen negative for past year marijuana use will be selected to participate in the study (n=450); all subjects who screen positive for past year use will be enrolled in the study (n=450). These 900 subjects will be stratified by gender, age, and past year marijuana use and randomized to one of three conditions: 1) computer-delivered brief motivational approach (BMI-C) (n=300; 150 non-users: prevention, 150 users: intervention); 2) therapist-delivered brief motivational approach (BMI-T) (n=300; 150 non-users, 150 users); or 3) a NIDA drug education booklet (DPB) (n=300; 150 non-users, 150 users). Primary outcomes will be evaluated at 3-, 6- and 12-months and include marijuana, alcohol, tobacco and other drug use. Key moderators of effectiveness will be examined, including behavioral intentions, self-efficacy, stage of change, school involvement, susceptibility to peer pressure, and potential health consequences (e.g., STD/HIV risk behaviors). This project will provide the critical first step toward the implementation of tailored marijuana prevention and intervention in primary care clinics.